These are step by step instructions on how to install VirtualBox 4.2 under Ubuntu. In the terminal window, type the following commands:

echo "deb http://download.virtualbox.org/virtualbox/debian $(lsb_release -sc) contrib" | sudo tee /etc/apt/sources.list.d/virtualbox.list
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get install virtualbox-4.2

Open VirtualBox from System Tools / Oracle VM VirtualBox, it should work now.

If you get an error: "Kernel driver not installed (rc=-1908)", use the following fix.

In the terminal window, type the following commands:

sudo apt-get install linux-headers-$(uname -r)
sudo /etc/init.d/vboxdrv setup

Enjoy!